1. Field of the Invention
The present invention relates to a novel photomask material.
2. Description of the Prior Art
Photomasks having high resolving power are necessary in the production of, for example, so-called integrated circuits (IC). As the photomask material for producing photomasks, a photographic material comprises a transparent support having coated thereon a silver halide photographic emulsion, in which material silver images are formed to prepare a photomask (hereinafter referred to as an emulsion mask), has been used. A large amount of this type of material is still being widely used.
However, an emulsion mask has many disadvantages in that the image layer is mechanically weak, the silver image is opaque to visible light as well, and the edge acuity is poor due to the grain property of silver.
Therefore, an excellent photomask without these disadvantages has been desired in this field. The so-called hard mask, represented by a chromium mask, has been developed to satisfy these requirements. At present, a chromium oxide mask, an iron oxide mask, a silicon mask, etc., are also known as well as the above-described chromium mask. These masks, except for the chromium mask, provide semi-transparent mask images (with these masks usually being called "see-through" masks) and these hard masks are extremely strong and durable. However, in comparison with the above-described emulsion mask, these masks possess the following defects. That is, since the hard mask material itself is not light-sensitive, a photoresist must be coated thereon to utilize a light sensitivity of the phtoresist, in order to prepare a hard mask having a hard mask material. However, the sensitivity of the photoresist is usually on the order of an ASA of about 10.sup.-5, which is far smaller than the sensitivity of an emulsion mask having a sensitivity on the order of an ASA of 10.sup.-2. This poor sensitivity requires a prolonged exposure time to expose this photomask material using a photo-repeater. Furthermore, a hard mask has the defect that the surface reflection of the mask is so high that light reflected on the surface upon imagewise exposure diffuses within the photoresist layer to reduce the resolving power. Another defect with a hard mask is that since the mask layer at the image area projects on the support, the mask layer is often chipped when the mask is pressed against a semiconductor wafer, with the mask image being worn away. This phenomenon naturally leads to a reduction in the life of the hard mask.
Therefore, the development of a photomask which possesses both the advantages of an emulsion mask (i.e., a high sensitivity and low surface reflection) and the advantages of a hard mask (i.e., high durability, see-through ability and good edge acuity) has been strongly desired.
Investigations have been made to apply the art relating photographic materials such as "Metalphoto", "Alphoto" or like trade named materials, to photomask use. Such a photographic material is prepared by subjecting the surface of an aluminum plate to anodic oxidation to form an aluminum oxide layer of a thickness of several microns to several tens of microns and filling the fine pores of about several hundred angstroms in size formed in the oxide layer with silver halide (hereinafter referred to for brevity as "aluminum photomaterial").
With this photographic material, the silver image can be formed by subjecting the material to development and fixing processing after imagewise exposure, in a manner similar to ordinary silver halide photographic materials.
As is well known, a photomask must have the property that the non-image areas are transparent to the light to which the photoresist is sensitive. However, since the above-described aluminum photomaterial has an opaque support of an aluminum plate, the material cannot be utilized as a photomask. Therefore, attempts were made to use a material comprising a transparent support having thereon an aluminum thin film layer, in place of the aluminum plate and to subject the thin film layer to anodic oxidation to thereby render the thin film layer transparent, as described in Japanese Patent Application No. 13478/73, corresponding to U.S. patent application Ser. No. 528,317 filed Nov. 29, 1974 now U.S. Pat. No. 3,961,962. The term "transparent" as used herein in the present invention means "transparent in visible and near ultraviolet light regions".
According to the method of the above-described application, an aluminum thin layer is provided on a transparent support by means of, for example, vacuum evaporation or sputtering, the aluminum thin layer is then anodically oxidized by contacting it with an electrolytic solution (under conditions such that the anodic oxidation rate of the aluminum thin layer is continuously reduced in the direction of from one portion of the layer to the other portion of the layer) to thereby render the thin layer transparent, and a light-sensitive silver halide is incorporated in a number of the resulting fine pores formed in the aluminum oxide thin film and uniformly distributed over the entire oxide film.
The photomask material obtained by this above described method provides good adhesion between the support and the oxide film, scarcely results in islands of aluminum remaining on the interface of the support and the oxide film, and further provides the following advantages in comparison with an aluminum photomaterial.
(1) With an aluminum photomaterial, an oxide film of a thickness of about 10.mu. is usually necessary for obtaining silver images with sufficient optical density, whereas in the above method a sufficient density is obtained with an oxide film having a thickness of about 2.mu.. A thin oxide film is advantageous for a photomask having high resolving power since any reduction in resolving power due to light-scattering in the oxide film is small.
(2) With a conventional aluminum photomaterial, silver halide is incorporated in the fine pores of the oxide film in an amount of about 5 volume % at the highest. The reason for this is if silver halide is incorporated in greater amount, an unexplainable reduction in the silver image density occurs. However, in the above-described method, the advantageous phenomenon that the silver image density increases as the amount of the silver halide incorporated in the fine pores increases has been discovered. That is, in the production of a conventional aluminum photomaterial, the following unexplainable phenomenon occurs: when, silver nitrate is first incorporated and then potassium bromide is fed into the fine pores to deposit silver halide grains therein and, after drying, this processing is repeated to further fill the pores with silver halide, the optical density of the silver images becomes lower than if the process of incorporating the silver halide is conducted only one time. If the process for incorporating the silver halide is conducted three times, the optical density of the silver images is further reduced. In contrast, in the above described method, the optical density of the silver images increases as the number of times that the process for incorporating the silver halide is conducted increases. This fact is completely opposite to the phenomenon which occurs with the conventional aluminum photomaterial. This is extremely effective for the purpose of obtaining silver images with a high optical density from a thin oxide film.
(3) With a conventional aluminum photomaterial, the shelf life of the photographic material is shorter than that of a normal silver halide photographic material having a silver halide emulsion layer as a light-sensitive layer, probably because of the presence of an aluminum layer under the aluminum oxide layer. Aluminum is quite active to silver halide and reduces silver halide to silver upon contact. However, in the above described method, an aluminum layer is scarecely present under the aluminum oxide layer, and hence the shelf life of the photographic material as described in Japanese Patent Application 13478/73, corresponding to U.S. patent application Ser. No. 528,317 filed Nov. 29, 1974 is extended.
As can be understood from the above descriptions, marked differences are present between the technique of the above-described method and a conventional aluminum photomaterial. However, it has been found that the photomask material obtained by the above-described method has the following disadvantages. That is, if the surface of the aluminum thin film provided on the transparent support is a mirror surface, a good photomask material is obtained; however, if the surface of the aluminum layer is not a mirror surface, in other words, is cloudy or tarnished, fine aluminum grains remain uniformly on the interface of the transparent support and the anodic oxide film. It has been found that the remaining aluminum grains correspond to the surface granular pattern of the aluminum thin layer. It becomes difficult to maintain the surface of the aluminum thin layer as a mirror surface, as the thickness of the aluminum thin layer increases. Empirically, a thickness of more than 2.mu. tends to result in a cloudy surface.
It is not preferred that a small amount of aluminum remains uniformly in a fine granular state on the interface of the transparent support and the oxide layer, since the transparency of the photomask is lowered.
It has been found that, in the above-described method, when the thickness of the aluminum thin layer is relatively small (e.g., less than about 2.mu.) and is a mirror surface, no aluminum remains in the form of small islands on the interface of the support and the oxide layer, but an aluminum film of thin uniform layer having a thickness of about 5 mm remains. Therefore, the transmittance of the oxide layer is reduced by a factor of about 10 to 20%, and further, there is the possibility that the silver halide in the fine pores is reduced during long storage.